The present invention relates to a cleaning device applicable to an electrophotographic copier, a laser printer, a facsimile apparatus and other electrostatic recorders for removing toner which remains on an image carrier, i.e. photoconductive element.
In an electrophotographic copier, for example, a latent image electrostatically formed on a photoconductive element is developed to become a toner image and, then, the toner image is transferred to a paper sheet and fixed thereon. Since some toner and paper dust remain on the surface of the photoconductive element after the transfer of the toner image to a paper sheet, the copier has to be equipped with a cleaning device for removing them before the next image is formed. One of various kinds of cleaning devices heretofore proposed includes a precleaning charger for discharging toner remaining on the photoconductive element, or residual toner, a fur brush for removing the residual toner from the photoconductive element, and a single flicker for beating down the toner from the fur brush penetrating into the latter. Another cleaning device known in the art includes a scavenger roller, or collecting roller, for collecting toner from a fur brush, and an elastic scraper for scraping off the toner from the scavenger roller. Usually, such cleaning devices further include a cleaning blade for removing that part of toner which fails to be removed even by the fur brush.
The residual toner remaining on a photoconductive element usually contains impurities, typically paper dust. When paper dust is caught between the scavenger roller and the edge of the scraper, the cleaning ability of the scavenger roller which acts on the fur brush is reduced resulting that the residual toner partly fails to be scraped off. Especially, since the toner is electrically insulative, the paper dust degrades the electrical effect of the scavener roller on the fur brush to thereby impair the toner removing ability of the fur brush, eventually causing a copy sheet to be smeared due to incomplete cleaning.
In a modern electrophotographic copier and others, the increase in copying speed is accompanied by the increase in the amount of toner and paper dust which remain on the photoconductive element and, therefore, adhere to the fur brush. Such an amount of residual toner and paper dust is beyond the ability of a single flicker which makes contact with the fur brush. Even the scavenger roller is incapable of fully collecting the residual toner and paper dust from the fur brush. In this situation, the residual toner and paper dust left unremoved from the fur brush are transported onto the photoconductive element again so that they reach the edge of the cleaning blade together with toner and paper dust which have not been removed by the fur brush. Since some toner is allowed to move past the cleaning blade and leak due to vibrations and others, the toner is in due course scattered at a location downstream of the cleaning device with respect to an intended direction of rotation of the photoconductive element. This part of toner adheres to the photoconductive element, an optical arrangement and others to lower the quality of images. Further, the paper dust wedged between the cleaning blade and the photoconductive element impairs the function of the cleaning blade and, thereby, causes black stripes to appear on a copy sheet while scratching the photoconductive element.
An implementation heretofore proposed for the effective collection of toner from the fur brush consists in applying a DC bias to the scavenger roller. However, a difficulty has been experienced in collecting the toner of the opposite polarity to the DC bias as well as paper dust. In the light of this, it has also been proposed to switch the polarity of the DC bias applied to the scavenger roller. This scheme is not fully satisfactory for the following reason. When, for example, a DC bias of positive polarity is applied to the scavenger roller, residual toner and paper dust which have been charged to the positive polarity cannot be collected although those charged to the negative polarity may be collected. When a DC bias of negative polarity is applied to the scavenger roller, toner and paper dust charged to the negative polarity fail to be collected. Such residual toner and paper dust left uncollected are allowed to adhere to the photoconductive element again, resulting in the previously stated problem.
Another problem with the fur brush is that while it is in contact with the photoconductive element and the flicker, its hairs are constantly affected by a force which tends to lay them in a particular direction corresponding to the rotating direction of the photoconductive element of the fur brush. Therefore, after a long time of use, the hairs of the fur brush become unable to regain their original position resulting that the outside diameter of the fur brush and, therefore, the degree or area of contact of the fur brush with the photoconductive element is reduced to aggravate the cleaning ability.
On the other hand, in a cleaning device of the kind described, an excessive cleaning ability, especially that of the fur brush, brings about another problem as follows. When the ability of the fur brush is excessive, the amount of residual toner and paper dust which is expected to reach the cleaning blade is extremely reduced. In this condition, the frictional force acting between the cleaning blade and the photoconductive element is intensified to allow the edge of the cleaning blade to wear soon. While the amount of residual toner transported to the cleaning blade may be increased by lowering the cleaning ability of the fur brush, such an approach would lower the total performance of the cleaning device. Another approach which may be contemplated to slow down the wear of the edge of the cleaning blade is reducing the pressure force which is exerted by the cleaning blade to the photoconductive element. Simply reducing the pressure force, however, would cause the cleaning blade to oscillate and produce noise. Although a lubricant or the like may be applied to the photoconductive element in a position upstream of the cleaning blade, i.e., between the fur brush and the cleaning blade, such a scheme is impracticable without complicating the construction. Further, to increase the amount of toner to reach the cleaning blade, an arrangement may be made such that a particular pattern provided in an ineffective image area of a glass platen is exposed and developed but not transferred to a paper sheet, i.e., it just contributes to the supply of non-transferred toner. This scheme, too, complicates the construction of the entire copier and causes the amount of development to scatter over a substantial range due to variations of the optical system and photoconductive element with time.